The invention disclosed provides a method for producing polymeric foams by stress nucleation, and for making new types of heterogeneous foams.
1. Background of the Invention
Microcellular foams have been developed for materials saving and reportedly have attractive mechanical properties. Small cells in the materials, which are normally associated with high cell density, are responsible for such properties. Microcellular foams are stronger than conventional foams which have much bigger cells; thus, providing a potential for a variety of applications.
2. Description of the Prior Art
Various techniques have been developed for producing microcellular foams, including batch processes and continuous processes. These processes are characterized by the following steps: saturating a polymer with a blowing agent, usually a gas; nucleating cells by suddenly reducing the pressure or increasing the temperature of the system; allowing cells to grow up to a certain size and then stopping further growth by rapid cooling. The saturation process in which a gas, driven by concentration gradient, diffuses into the polymer is achieved by exposing the glassy or rubbery polymer to a compressed gas. See, for example, U.S. Pat. No. 4,473,665 issued on Sep. 25, 1984 to Martini-Vvedensky et al.; U.S. Pat. No. 5,223,545 issued on Jun. 29, 1993 to Kumar; U.S. Pat. No. 5,670,102 issued on Sep. 23, 1997 to Perman et al.
Cell nucleation occurs when the gas saturated polymer is in the rubbery state, i.e. when the foaming temperature is higher than the glass transition temperature (Tg) of the polymer-gas system. The pressure drop-induced nucleation is achieved by simply releasing the system""s pressure while the polymer is in the rubbery state. See Goel and Beckman,1 and Baldwin et al.2 The temperature increase-induced nucleation is achieved by heating the gas saturated polymer from its glassy state to a temperature where the system goes into the rubbery state or by heating the gas saturated polymer already in the rubbery state to a higher temperature. See U.S. Pat. No. 4,473,665 issued on Sep. 25, 1984 to Martini-Vvedensky et al. and U.S. Pat. No. 5,334,356 issued on Aug. 2, 1994 to Baldwin et al. The nucleation occurs due to the thermodynamic instability caused by the pressure drop or temperature increase, because the equilibrium gas solubility decreases with decrease in pressure or increase in temperature. Cell growth starts instantaneously after the nucleation step. The common way to restrict or stop the cell growth is to reduce the system""s temperature by rapid cooling.
Nucleating agents are usually added to the system to aid nucleation. This approach has proven very successful in producing conventional foams. In the production of microcellular foams, however, nucleating agents are seldom used because the required cell density is quite high and the contribution from nucleating agents to that end is insignificant. Nevertheless, a second phase polymeric material (U.S. Pat. No. 5,369,135 issued on Nov. 29, 1994 to Campbell and Rasmussen) was found to provide nucleation sites to foam a polymer within a certain cell density range, although such a second polymer phase did not show any tendency to facilitate foaming at lower temperatures.
The nucleation step is very important for achieving a desired cell density and cell size. In conventional pressure drop-induced or temperature increase-induced methods, nucleation occurs at quite high temperatures at which cell growth is fast and, thus, difficult to control. In an extrusion process, for instance, cell coalescence due to the uncontrolled cell growth can occur, resulting in poor quality foams. See Behravesh et al.3 
Nucleating cells by applying stress is a new concept. It has been reported by Lee4, that application of stress enhances the effect of nucleating agents in the processing of conventional foams though the applied stress itself does not nucleate cells. The present invention, however, provides a method to nucleate cells directly by the application of stress.
Whatever the nucleation method used in a given foaming process, the majority of man-made polymer foams are isotropic (i.e. homogeneous) in structure and, therefore, quite uniform in properties. The reason is that the conventional pressure drop or temperature increase nucleation methods subject the entire polymeric body to thermodynamic instability. Foaming thus tends to develop throughout the polymer giving a regular and uniform cellular structure. On the other hand, natural materials, such as bones, woods, and corks, have anisotropic foam structure. That is, the cellular structure is not uniform or regular throughout the material, or the structure may be regular along one direction but not in another direction. Such a special texture gives natural materials wonderful properties leading to a variety of uses whether as structural materials or as functional materials.
Accordingly, it is an object of this invention to provide a new nucleation process that can be used at low as well as high temperatures and allows for a better control of cell growth.
It is a further object of the present invention to provide a new nucleation method that can be adapted in the existing foaming processes and produce foams which otherwise are difficult or impossible to produce by such processes.
It is another object of this invention to provide a new nucleation technique that can be used to design and produce anisotropic foams, enriching the applications of man made foams.
Thus, a polymer saturated with blowing agent can be stressed e.g. by mechanical or hydrostatic pressure means, at a certain temperature to produce foam. The cell nucleation arises from the thermodynamic instability caused by the stress. Stress can be applied by various ways, of which compression is preferred and is easily achieved e.g. using a press or a rolling system. The compression mechanism can be adopted for both batch and continuous processes. Stress-induced nucleation has short induction time and develops almost instantly throughout the stressed material, giving foams with high cell density, small size cells, and the cellular characteristics can be easily controlled in a prescribed way. The method can be widely used for producing a variety of microcellular foams and can be extended to produce conventional foams.
The polymer to be foamed can be in any desired geometrical shape e.g. a preformed sheet or formed into a sheet by conventional molding techniques. Typically, saturation time will depend upon the polymer-blowing agent combination used and the geometrical characteristics of the polymer. More specifically, there is a finite time, which is required for the polymer to become saturated with the blowing agent, which will vary depending upon the surface area to volume ratio of the polymer.
According to one aspect of the invention a method is provided for producing a closed cell polymer foam, comprising
(a) selecting a suitable polymer and inert blowing agent combination, wherein the polymer is in a solid or melt state, and the blowing agent is in the form of a gas or a volatile liquid,
(b) exposing the polymer to the blowing agent at a conditioning temperature, pressure and exposure time, selected according to the thermodynamic properties of the polymer/blowing agent combination to provide a polymer/blowing agent solution having a desired solubility up to a maximum of saturation solubility of the blowing agent in the polymer,
(c) slowly depressurizing to ambient pressure to prevent premature foam formation,
(d) applying an external stress to the polymer-blowing agent solution at a temperature at which the polymer was conditioned with the blowing agent up to about the Tg of the neat polymer, wherein the amount of stress applied is dependent upon the thermodynamic properties of the polymer/blowing agent combination and the amount of blowing agent dissolved in the polymer, to form the foam, and
(e) quenching the foam by rapid cooling to a lower temperature.
Also according to the invention stress nucleation can also be used for manufacturing speciality foams. One of its special applications is in producing anisotropic (i.e. heterogeneous) foams with potential applications as structural and/or functional materials. Such foams provide a good solution for materials saving while delivering the desired properties. For example, a polymer containing dissolved blowing agent can be stressed in selected areas only to produce a heterogeneous material where the selected areas are foamed and the rest are not. A polymer can also be conditioned such that only the surface layer contains the blowing agent and the remainder of the material contains none or only an insignificant amount of the blowing agent. The thickness of the surface layer containing the blowing agent will depend on the conditioning parameters such as exposure time, temperature, and pressure of the blowing agent. The polymer surface can then be stressed to produce a heterogeneous foam where the interior of the polymer is not foamed and only the surface layer or a certain fraction of the surface layer is foamed.
Accordingly, another aspect of the invention involves a solid polymer having a heterogeneous morphology, comprising
(a) a solid polymer, and
(b) a modified portion of said polymer, the modification being selected from the group consisting of
(i) a portion of the polymer being foamed, the remainder of the polymer left unfoamed, and,
(ii) a portion of the polymer being foamed to a certain degree, the remainder of the polymer being foamed to a different degree.